Small second acoustic peak from interacting cold dark matter?

Abstract

We consider the possibility of explaining the observed suppression of the second acoustic peak in the anisotropy spectrum of the cosmic microwave background (CMB) by interaction between a fraction of nonbaryonic cold dark matter (CDM) and normal baryonic matter. This scenario does not require any modifications in standard big bang nucleosynthesis. We estimate the required values of the cross-section-to-mass ratio for elastic scattering of CDM particles off baryons. In the case of velocity-independent elastic scattering (in the velocity interval $\ensuremath{\upsilon}\ensuremath{\sim}{10}^{\ensuremath{-}5}\char21{}{10}^{\ensuremath{-}3})$ we find that such particles do not contradict observational limits if they are heavier than $\ensuremath{\sim}{10}^{5} \mathrm{GeV}$ or lighter than $\ensuremath{\sim}0.5 \mathrm{GeV}.$ Another candidate, which may appear in the models with infinite extra dimensions, is a quasistable charged particle decaying through tunneling into extra dimensions. Finally, a millicharged particle with an electric charge ranging from $\ensuremath{\sim}{10}^{\ensuremath{-}4}$ to $\ensuremath{\sim}{10}^{\ensuremath{-}1}$ and with a mass $M\ensuremath{\sim}0.1 \mathrm{GeV}\char21{}1 \mathrm{TeV}$ also may be responsible for the suppression of the second acoustic peak. As a by-product we point out that CMB measurements set new limits on the allowed parameter space for the millicharged particles.